Turbomachines such as gas turbine engines used for propulsion and power generation employ arrays of flow directing elements such as rotating blades and nonrotating vanes to exchange energy with a working medium fluid, typically air, flowing through the engine. Each blade and vane has an airfoil portion which can be susceptible to flutter. Flutter is a self excited aeroelastic phenomenon in which airfoil vibration is induced by coupling between the smoothly flowing working medium and the elastic deformation of the airfoil. The aerodynamic forces exerted on the airfoil add energy and increase the vibratory amplitude and stresses during each vibration cycle. In extreme cases, the vibratory amplitude and stresses become large enough to cause structural failure of the blade or vane. Although flutter can occur in both blades and vanes, experience shows that the airfoil of a rotating fan blade is particularly susceptible to flutter.
One factor which contributes to flutter susceptibility is the highly uniform character of blades and vanes resulting from the modern manufacturing and quality assurance techniques used in their production. Uniformity is generally considered to be desirable since it reduces the likelihood that a blade or vane will have to be discarded or reworked due to violation of manufacturing tolerances. Unfortunately, uniformity of blades and vanes in an array can also reinforce any tendency of an airfoil to flutter during engine operation. This is particularly true if, as is customary, the circumferential spacing between adjacent blades or vanes is also uniform.
Flutter susceptibility as described above is greater in newly manufactured engines, or engines which have been refurbished with new blades or vanes, than in engines which have accumulated numerous hours of service. Uniformity induced flutter susceptibility diminishes during normal service because the blades and vanes experience erosion and other minor mechanical distortions which introduce enough variability to substantially eliminate airfoil flutter. However it may take many hours of engine operation to accumulate sufficient variability or nonuniformity to preclude flutter. Therefore it is desirable to intentionally introduce flutter resisting variability or nonuniformity into the blades and vanes of new and refurbished engines. The introduced nonuniformity ensures flutter resistance during the first several hundred hours of operation.
It may also be the case that an engine manufacturer has produced engines which, due to the higher variability inherent in older, less efficient manufacturing processes, have never been susceptible to flutter. From time to time a small number of blades or vanes in an array may be rendered unserviceable due to damage sustained during operation. The replacements for the damaged blades or vanes may be newly manufactured blades or vanes having flutter resisting nonuniformity. Therefore the compatibility between newly manufactured blades and vanes, which incorporate intentional nonuniformity to resist flutter, and existing blades and vanes, which do not include such nonuniformity, must be ensured.
It is also important not to compromise the aerodynamic performance of blade or vane arrays by the inclusion of flutter resisting nonuniformity.
The provision of nonuniformity in a blade array (in the context of rotating stall avoidance rather than flutter avoidance) is addressed in U.S. Pat. No. 3,536,417 issued to Stiefel et al. Stiefel teaches, among other things, the use of a varying angle of incidence on every other blade in an array. However no specific means for achieving such alternation is taught. Stiefel also teaches cutting back the leading edge of selected blades to achieve circumferential nonuniformity. This approach may be unattractive for modern gas turbine engines since their airfoil contours are complex and the machining operations are correspondingly time consuming and expensive. Moreover, the aerodynamic performance of modern engines may be compromised by such variations in the leading edge geometry.
In U.S. Pat. No. 4,863,351, Fischer et al. teach the provision of circumferential nonuniformity (in the context of noise control) by using blades whose longitudinal blade axes are oriented differently from the neck axes to effect different sweep angles of the airfoils. However this approach suffers from the disadvantage that a damaged blade must be replaced with one having the same blade axis orientation so that the balance of the rotor is preserved. As a consequence, multiple types of blades must be manufactured and maintained in inventory. Economy of scale in manufacturing is thus sacrificed and spare parts inventory management is complicated and made more costly.
What is needed is a blade or vane which incorporates intentional variability to improve flutter resistance without compromising aerodynamic performance or compatibility with existing blades or vanes which do not have such variability. Moreover, economy of manufacturing and ease of inventory management must be ensured.